Silicon Chip 200Watt LD amplifier

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#281
hi jp_howard,

Thanks for the clarification. Strange that the Duet has this problem. My Squeezebox is the same as my Tranporter.

Was the Duet problem the same with Hugh's Duet? Who's PSU? Bad switchmode supply?

It's good to do your own experimenting with different types of amps because there is so many conflicting opinions.

regards
 
#282
Amp Test

jp_howard said:
Well, interesting developments with this 'hiss' issue... I heard back from Lawrence Chan, who makes the DAC, and he said in his two years of selling it this is the 1st he's heard of any HF hiss. I've tried the DAC with 2 other amps, and I don't hear any hiss at all.

So even although the hiss isn't being produced by this amp, it does seem that it's particularly sensitive to it. I've also testing my Duet's internal DAC with a couple of other amps, and although some hiss is audible in all of them, it's far worse with the SCULD than with the other two.

So I'd say that, at least in the standard spec (and with the tweaks suggested so far), the SCULD is only a good choice if you've got a very low-noise source, or if you're not too sensitive to HF noise.

BTW, I've tried increasing both the 820pf cap and 100R, but couldn't get rid of the hiss without destroying the SQ.
Click to expand...

Hello jp_howard,

If you want you amps performance to be measured I can measure its THD etc , along with other tests that can be done this might shed some light on your problem. Send us an email , I live in Melbourne, Hugh knows me.

That you say it is sensitive to certain sources seems a bit strange.
I must confess I have heard expensive equipment that was noisy to the ear, this is bad as all solid stuff with CD source should be very quite.

Regards
Arthur
 
#283
Yes Hugh's Duet also had a HF hiss. That's using an upgraded power supply. However, I don't think the hiss is related to the power supply, since so much (but not all) of the hiss goes away when using an external DAC. So I figured it's a DAC issue.

I should mention that I seem to be fairly sensitive to HF - I'm always the first one in a room to notice the phone ringing in a distant part of the house, and the first to complain about high-frequency noise in an environment. So part of the problem here may be that I'm just fussy!...
 
#285
Jeremy
It would be interesting to know if this is random noise when idling, but goes away when there is a signal. T o find this out you would need a test CD with a silent track on it. I noticed that there was a patent filed to fix this supposed problem.
Alex
 
#286
Interesting Alex - it would never have occurred to me to try that!

Anyhoo... I just tried a silent track, and the level of hiss was unchanged when the track was playing.

I also tried a FLAC of a Beethoven Piano Sonata at very low volume (a piece I know has some silent bits, which sound really silent when connected to my Transporter), and the hiss was the same during the silent bits.
 
#287
IIRC (edited Just found the book)
The old SCULD100 had an input impedance of ~18k ohms.
The SC20W has an input impedance of ~10k
The SC200W has input impedance of ~12K

As stated in the SC article,
"Both the bias resistor for Q1 andthe series feedback resistor to the base of Q2 are set at 12kohms (instead of 18k in the original Ultra-LD and Plastic Power designs), again to minimise source impedance and thereby, Johnson noise."

I didn't notice any noise from the SC100W.
Maybe the lower impedance is the problem....
The reason SC made a preamp for it.😡





Most commercial gear is designed for 47k input impedance.
 
#290
Hi
To all you guys down there in Australia, I have a question, the next time any of you go down to your Local Jaycar Store could you check if they have any SCLD 200 kits in stock .
The website says out of stock & i need 2 more kits so i can build a another pair of amps with all the suggested mods , i dont want to "hack " my existing amps as they are running so good , this way i can compare the original design with the new "improved"version just by swapping modules.
Unfortunately up here in Canada it will take 2-3 weeks to get the new kits once i can order them.
Any help appreciated..

Regards MickyH
 
#293
Hi all,

If there is enough interest, I can place an order through Electus Distribution (assuming they really have stock). As you can see the price is good. It's a an hour and half round trip to the distributor so a minimum of 6.

RRP: $94.95

Ultra-Low Distortion 135WRMS Amplifier Module
STOCK-CODE: KC5470

Qty 1+ AU$60.77
Qty 3+ AU$56.97
Qty 5+ AU$55.07

plus 10% GST (AU$5.50)
plus postage (no idea)

This ultra low distortion amplifier module uses the new ThermalTrak power transistors and is largely based on the high-performance Class-A amplifier which was featured in SILICON CHIP during 2007. The ThermalTrak transistors are a new version of the premium MJL3281A & MJL1302A and have an integral diode for bias compensation. As a result, the circuit has no need for a quiescent current adjustment or a Vbe multiplier transistor and has an exceptionally low distortion figure. Kit supplied with PCB and all electronic components. Heat sink and power supply not included.
• Output power: 135WRMS into 8 ohms and 200WRMS into 4 ohms
• Frequency response at 1W: 4Hz to 50kHz
• Input sensitivity: 1.26V RMS
• Input impedance: ~12k ohms
• Harmonic distortion: <.008% from 20Hz to 20kHz
• Signal-to-noise ratio: 122dB unweighted (22Hz to 22kHz)
• Damping factor: <170 at 100Hz
• Stability: Unconditional

regards
 
#295
awpagan said:
As stated in the SC article,
"Both the bias resistor for Q1 andthe series feedback resistor to the base of Q2 are set at 12kohms (instead of 18k in the original Ultra-LD and Plastic Power designs), again to minimise source impedance and thereby, Johnson noise."

I didn't notice any noise from the SC100W.
Maybe the lower impedance is the problem....
Click to expand...
Is it safe for me to try changing these resistors to a higher value? Or would that require changing other components too?
 
#296
jp_howard said:
Is it safe for me to try changing these resistors to a higher value? Or would that require changing other components too?
Click to expand...


I just turned up the volume (full tilt on the preamp) with pc sound connected and nothing playing.

A bit of hiss audible from speakers about 1 to 2 feet away but not necessarily so.

I need to do some further tests, but this is standard input values.
Also this is from a prelude sound card via spdif to dac > preamp > amp.
Dac and preamp are direct coupled(no caps) only cap is on the amp input.

Speakers are TL6's (with some changes)
89.5dB 1W@1M (in standard form, same or bit more now)
http://www.theloudspeakerkit.com/shop/productdetails.asp?ProductID=71&ProcessType=1

What is the volume set at when you can hear the noise?
Pc digital volume or analogue volume?
 
#297
jp_howard said:
Micky: If you're keen to get them fast, lemme know, and I'll see if my local Jaycar has a couple - then I can pop them in the post tomorrow. Do you need the PSU modules too? Toroidal transformers?
Click to expand...

Hi JP
Thanks for the offer , i only need the 2 Amp Kits , as i already have suitable power supplies, transformers etc to build a 2nd amp.
I think that shipping by Airmail would be about $50 at least thats what Jaycar charged me for my first 2 Kits.
If you dont mind doing this let me know the Total $$$$ , i can pay you immediatly by PayPal if that would be OK..

Regards Mick
 
#298
I don't have a preamp, so my amp is at full volume - only volume changes are on the source. I don't get any noise with the amp on alone, or with it connected to the Transporter - only with a Duet or the external DAC.

I had another guy listen last night, and he couldn't hear it. So I've now checked with 4 people, none of whom could hear the hiss with the external DAC at all. I guess that explains why Lawrence hasn't received any complaints - it's quiet enough that most people can't hear it!

With the Duet connected directly however, the hiss is very clear and everyone I've asked can hear it.
 
#299
SC ULD2

Jeremy
As an aside, I am able to use digital volume control from my OppoDV 981HD player via SPDIF into my Class A amplifier directly, but unless the original material is extremely good, or 24 bit instead of 16bit, then any more digital attenuation than around 10dB considerably degrades the sound. This is not the case with analogue . I would love to go directly from my X-DAC V3 and use this route, as the sound from high resolution sources is absolutely stunning. Unfortunately it isn't worthwhile with most material.
Alex
 
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